1. Field of the Invention
This invention pertains to the field of optical corrective surgery and more particularly to a system and method for monitoring aberrations of the eye in real time during optical corrective procedures that operate such as to leave the optical zone of the eye unobscured during the procedure.
2. Description
Several technologies have recently be developed for laser corneal surgery for correction of refractive error. These include Laser in-situ Keratotomy (Lasik), Photo-refractive Keratotomy (PRK), Redial Keratotomy, laser thermal keratotomy (LTK), and Laser Thermal Drying (LTD). These technologies use a laser either to remove material or to modify the structure of other properties of the cornea in order to make the desired changes in refraction. In the case of Lasik, a small flap of corneal material is cut and peeled back to allow ablation of the underlying material, rather than the outer epithelial layer. PRK directly ablates this outer layer. LTK and LTD apply an infrared laser to the outer periphery (outside the optical zone) to change the structure and shape of the cornea to achieve the desired refractive correction.
In each of these different technologies, the hydration state of the eye, the healing response of the particular subject, the bio-mechanics of the retina and many other factors directly contribute to the results. Furthermore, the refraction must be achieved through a pre-computed process that accounts for all of the various factors. The goal is to provide a process that achieves the desired change of refraction while minimizing the optical aberrations of the ocular system. However, in many cases the refractive surgery itself introduces significant aberrations. This may be due to either a change in the bio-mechanical structure, or merely due to the edge of the treatment zone.
A number of instruments have been developed that have served as diagnostics on this process. These includes subjective and auto-refraction, corneal topography, pachymetry, and wavefront aberrometry. Of these, only wavefront aberrometry directly measures the optical aberrations of the full optical system. Pre and post diagnostics using these advanced instruments have provided a tremendous amount of information that has lead to the improvement of the various methods.
However, there are still variables that cannot be properly monitored that affect the outcome. In particular, factors like the hydration state of the eye and healing response are difficult to account for in advance. The ablation profile for Lasik has long been known to be non-linear and have a different strength for positive or negative corrections. The ablation algorithms have been developed to take this into account, but there is still considerable variation from subject to subject because of unknown factors.
If the diagnostics could be applied in real time, during the refractive surgery, then some of this variation could be removed. This would allow the laser surgery to operate in a “closed-loop” mode, with the amount of refractive modification being monitored and controlled during the procedure. While it is possible to monitor the change in shape of the cornea in “real-time” with corneal topography or other surface means, this only indirectly affects the total optical path and hence the refraction and higher order terms.
Accordingly, it would be desirable to provide a system and method for monitoring the aberrations of the eye in real time during Laser Thermal Keratotomy, Laser Thermal Drying and other laser ablation refractive surgery procedures that operate such as to leave the optical zone unobscured. It would also be desirable to provide a system and method for using wavefront aberrometry to monitor the refractive surgery process in real time to give a signal that allows for “end-point” detection, i.e., to provide a real-time signal to the laser system to stop the procedure when the desired correction has been achieved. It would further be desirable to develop a set of nomograms for each individual aberrations, which may be described by Zernike polynomials, so that the laser profile or procedure can be adjusted in real-time to minimize the induced aberrations.
With LTK and LTD the optical zone is itself not directly modified and the refractive operation of the eye remains functional during the treatment. Thus it is possible to incorporate the diagnostics directly into the laser refractive system without obscuration or unanticipated modification of the optical zone. Wavefront aberrometry provides the added benefit of being able to monitor the effect of the refractive surgery on both the desired refraction terms (focus and astigmatism) along with various high order aberrations. Since these procedures operate near the edge of the optical zone, it is important to consider these effects. Furthermore, wavefront aberrometry directly monitors the total optical aberrations of the full system.
It should be noted, however, that there is a significant difficulty with incorporating the diagnostics into the lasers that are used for Lasik or PRK. In the case of Lasik, a cut is made through the cornea and the loose flap of corneal tissue is pulled away so the Lasik laser can ablate the underlying tissue. This means that the measurement made by any diagnostic instrument during the ablation would not be measuring the actual optical path that will result when the corneal flap is folded back down. Also, the cutting of the cornea releases the tension in the corneal fibers and that tension is not restored when the flap is laid back down. So the resulting optical performance of the cornea is not what is was before the cut was made. While it may be possible to calibrate for these effects, it certainly falls short of the goal of directly measuring the desired result in real-time during the procedure.
Frey, Burkhalter, Zepkin, Poppeliers and Campin in U.S. Pat. Nos. 6,271,914 and 6,271,915 introduced a method for ablating corneal material while monitoring the process in real time using a Hartmann plate sensor. Unfortunately, their techniques rely on modifying directly the optical zone that is measured. During the Lasik or PRK procedures that use ablation of portions of the cornea, the process of ablating material leads to unknown and undetermined optical scattering and effects during the ablation process. The surface of a dry cornea (needed for properly controlled ablation) or the interior surface that is exposed during the Lasik procedure are inherently rough. Thus these surfaces would scatter the injected and reflected light that is used for monitoring the wavefront. This significantly degrades the quality of the information obtained, making the aim difficult to achieve.
However, there are certain types of laser systems that do not directly modify the optical zone so as to affect the measurement. These include the Laser Thermal Keratotomy, Laser Thermal Drying, and femto-second laser systems. With these systems it is possible to implement a method for monitoring and controlling the optical refraction in real-time through the use of this invention.
The present invention comprises a system and method for performing optical corrective procedures with real-time feedback.
In one aspect of the invention, a system for adjusting an optical characteristic of an eye includes a refractive surgery instrument adapted to perform a procedure to modify refraction of an eye, an objective diagnostic apparatus adapted to measure at least one of the refraction of the eye and an aberration of the eye while the procedure is being performed, and an aperture-sharing element adapted to inject a refractive surgery beam and a monitoring diagnostic beam into the eye.
In another aspect of the invention, a method of adjusting a refraction of an eye, includes: performing a procedure to modify the refraction of the eye; while the procedure is being performed, measuring the refraction and/or an aberration of the eye; and terminating the procedure when a change in the measured refraction and/or the measured aberration reaches a desired value.